Generally a light-emitting diode (LED) package comprises a carrier (e.g. a leadframe), an LED chip disposed on a surface of the carrier, and an electrically-conductive wire for connecting the LED chip to the carrier. Each type of LED chip is only capable of emitting radiation of a narrow wavelength range in the frequency range of visible light within the electromagnetic spectrum—specifically, blue LED chips only emit radiation of a narrow wavelength range of around 450-475 nm that is normally perceived as ‘blue’ light; red LED chips only emit radiation of a narrow wavelength range of around 620-740 nm that is normally perceived as ‘red’ light; and green LED chips only emit radiation of a narrow wavelength range of around 520-570 nm that is normally perceived as ‘green’ light. In order to emit radiation of a wider wavelength range, an LED package may include a photoluminescent mixture comprising a photoluminescent material capable of absorbing a portion of the radiation of a certain wavelength range emitted from an LED chip and re-emitting radiation of a different wavelength range. The radiation re-emitted from the photoluminescent mixture then combines with the unabsorbed portion of the radiation emitted from the LED chip to emit radiation of a desired wavelength range. For instance, an LED package—which is capable of emitting perceived ‘white’ light—comprises a photoluminescent mixture having a photoluminescent material, such as phosphor, that absorbs a portion of blue light emitted from an LED chip to re-emit yellow light. The yellow light re-emitted from the photoluminescent mixture then combines with the unabsorbed blue light emitted from the LED chip to emit the perceived ‘white’ light from the LED package.
A problem in using a photoluminescent mixture to fabricate LED packages is the material changes of the photoluminescent mixture that affect the wavelength range of the re-emitted radiation. Such material changes of the photoluminescent mixture relate to the settling effect of the photoluminescent material in the photoluminescent mixture towards the bottom of the LED chip, which affect the ability of the photoluminescent mixture to absorb a desired amount of radiation emitted from the LED chip. This consequently affects the desired amount of radiation to be re-emitted from the photoluminescent mixture such that light of a desired colour is not emitted from the fabricated LED packages.
Thus it is an object of the present invention to seek to reduce the material changes of a photoluminescent mixture, so that the wavelength range of light that finally emits from an LED package comprising the photoluminescent mixture is more consistent to produce the desired colour.